(a) Field of the Invention
The invention describes devices and processes including lateral electrical contact outlets on lithium sheets which are used as anodes in lithium generators consisting of at least one. multilayer assembly of thin electrode films and polymer electrolytes in wound or stacked form. The patent describes materials and devices including a contact outlet on thin sheets of lithium in the vicinity of the plastic materials of the generator as well as procedures for producing these devices. The claimed lateral contact outlet devices are particularly suitable for all solid polymer electrolyte generators because they are very slightly resistive, they are adapted to the chemical reactivity of lithium and its alloys and are capable of ensuring an efficient heat exchange between the generator and its external casing. In one of the preferred devices, the electrical contact on the thin lithium sheets is obtained with a compatible metal, preferably copper, iron, nickel of alloys thereof, directly applied on lithium. A variant of this device consists in first providing an intermediate metallic layer of a lithium base metal or low melting lithium alloys which is applied in the form of a compact deposit at the end of the sheets of lithium. This deposit, called intermediate metallic layer, thereafter enables to obtain a second electrical contact, on its other face, with an inert and rigid metal, which is compatible with lithium, and is capable of maintaining the quality of the electrical contact between the anode of the generator and the external casing in spite of a possible superficial oxidation of lithium or its alloys.
The application of a metal such as copper which is in direct contact with lithium enables, whenever possible, to use the generator in dry air and additionally contributes to facilitate a thermic transfer between the latter and its external casing. The alternative which consists in using a metal such as lithium or its low melting alloys to provide an intermediate metallic layer, solves the problem of chemical reactivity with the lithium anode, facilitates the self-welding and cohesion of the deposit and ensures some deformability of the contact zone during thermic and electrochemical cycles of the generator. On the other hand, the low melting point of the filler metal facilitates its application on the edges of thin lithium films, even at a short distance from the other plastic components of the generator: insulating support films of lithium, electrolyte and composite cathode. The invention includes preferred embodiments and also describes means to obtain conductive and cohesive metallic deposits for the metallic layer which is compatible with lithium as well as for the lithium base intermediate layer. The quality of the weldings obtained according to the embodiments of the invention, between lithium and its alloys and certain hard and compatible metals is sufficient to preserve the electrical contacts of the anode from an oxidation of the surface of lithium by the surrounding gaseous phase. Another advantage of the devices according to the invention is to ensure an efficient heat transfer between the sheets of the generator and the outer casing of the latter. This aspect is particularly important for the safety of polymer electrolyte lithium generators where no free liquid electrolyte is present to facilitate heat exchanges between the generator per se and its external casing.
(b) Description of Prior Art
The development of primary and rechargeable lithium generators, has been on the increase during the latter years following an increasing demand for dense and light sources of energy. The important density of energy and the remarkable properties of preservation of lithium batteries give them a noted advantage over the other available systems which operate in aqueous media. However, a generally high manufacturing cost, a power which is sometimes limited at low temperature as well as consideration of safety with respect to the use of lithium still limit their use to small batteries and specialized markets.
One way to remedy these limitations consists in replacing the liquid organic electrolytes presently used in lithium generators by thin films of polymer electrolytes generally consisting of polyether complexes and lithium salts. It is known that plastic films may be prepared rapidly and with large surfaces, by means of automated processes, in the form of thin films of the order of a few micrometers thick. These films, which are cheap to produce, enable, in principle, to produce large size and high power generators by a mere increase of the surface of the generator in the form of thin films. On the other hand, the preparation of an all solid generator by using non-fusible solid polymers, instead of organic liquids, enables, in principle, to produce a safer system because it is more susceptible of limiting the speeds of reactions of the chemical reactants with one another or, in case of accidental exposure to ambient air or water. The polymers which are capable of being used in such solid state generators have been described in previous patents (U.S. Pat. Nos. 4,303,748; 4,578,326 and 4,357,401) as well as ways of assembling them (U.S. Pat. Nos. 4,897,917; 4,824,746 and French Patent No. 87 08539).
An increase of the active surface of lithium generators when polymer electrolytes are used is however met with the difficulty of developing equivalent surfaces for the current collectors of the anode and the cathode. A practical solution consists, for example, in the case of the cathode, to use aluminum and, in the case of the anode, to use the sheet of lithium per se as current collector. This approach is sometimes used in coiled organic liquid electrolyte generators, for example in AA, C or D formats; in this case, the anode consists of a film of lithium having a thickness of about 130 micrometers (.mu.). At such a thickness, lithium is sufficiently resistant to be freely handled by means of assembling machines and the collection of current from the anode is then ensured through the end of the sheet of lithium or, if needed, by means of transverse metallic tongues which are fixed to the film of lithium at regular intervals in order to reduce the ohmic drop in the collector. This solution is difficult to transpose in the techniques used for polymer electrolytes generators which use much thinner assemblies and which require lithium thickness between 40 and 1 micrometers. At these thicknesses, the films of lithium are much less mechanically stable and should be supported (e.g. U.S. Pat. Nos. 4,824,746 and 4,897,917) in order to be handled by assembling machines. The limited electrical conductivity of thin metallic lithium prevents on the other hand, in the case of coiled batteries, to collect the current which has accumulated at the end of the coil since the length to be drained is substantial and causes in a substantial ohmic drop in the collectors. This limitation, which is due to the thinness of the films and the lengths to be used in a technique based on ultra-thin films, therefore imposes a lateral collection of the coiled device in order to reduce the distance to be collected. This observation is also true in the case of generators which are made by stacking discontinuous thin batteries or are mounted in zig-zag in order to reduce ohmic drops. A known way to ensure lateral collection consists in applying transverse conductive tongues at regular intervals of the coiled anode or cathode in order to reduce the length to be drained.
However this possibility is hardly suitable for very thin films (local over thickness or low mechanical property of the tongue). Another possibility consists in laminating the anode of lithium on a thin inert metallic collector thereby enabling a lateral collection, through conventional processes of welding, on the inert collector. However, this additional metallic collector for the anode has been found to be extremely damaging in terms of weight and cost. By way of example, the cost of nickel or copper sheets, which are compatible with lithium, is about 1$/ft.sup.2 at the required thicknesses (e.g., 5-10.mu.).
The manufacture of ultra-thin capacitors including metallized plastics by pulverizing a lateral collector on the edge of the coiled films represents a more interesting model for the technique of assembling lithium batteries based on polymer electrolytes. This type of capacitor generally consists of two identical insulating plastic films (polypropylene or polyester, about 3 to 30 micrometers) which are metallized on one face, with the exception of one non-metallized lateral band, and are co-wound with a slight offset so as to be able to collect each of the films at one opposite end by means of a metallic deposit applied on the metallized end of each of the two films. The electrical contacts used in these devices are generally based on zinc, aluminum or silver applied in the form of conductive pastes including an organic binder or in the form of deposits obtained by pulverizing: by flame spray or with an electrical arc (or shooping) in the case of zinc and aluminum. The latter type of contact outlet, known in the industry of capacitors, is described in the European Patent Application published under number 0073555 and French Patent Application published under number 2,589,620.
It has been observed experimentally that these types of assembly and lateral contact outlet, which are compact, rapid and economical may be adapted to polymer electrolyte generators when inert metallic collectors are used, for example, when the collector for the cathode is aluminum. Up to now, these processes would not seem to be easy to directly transpose to the collection of lithium anodes consisting of thin lithium films for the following reasons:
the pulverization of zinc by flame spray used in capacitors is not compatible for lithium generators because of the release of water due to the combustion; PA1 the compositions of silver or zinc powder, generally based on organic binders of the epoxy type are not chemically stable in the presence of lithium, particularly at high temperatures; PA1 the chemical reactivity of lithium prevents the use of known metals such as zinc and aluminum and their alloys which are normally used for pulverization under an electrical arc (shooping) during the manufacture of the capacitors. As a matter of fact, it has been experimentally observed that these metals, react spontaneously with lithium to give hard and friable inter-metallic compounds which prevent the formation of a slightly resistive and reliable electrical contact; PA1 the metals which are compatible with lithium such as nickel, iron, copper, molybdenum, etc. have very high melting points and for this reason appear to be hardly applicable directly by vaporization on a multilayer assembly of lithium films and plastic materials. By way of example, tests made by the Applicant with a commercial device for plasma pulverization (Plasma Spray) with a Medco device (Division of Perkin-Elmer) Model MBN using nickel or copper powder as coating metal, show that there is an important heat degradation of the plastic films, which are PP insulating material and polymer electrolyte of the generator, when the metal is projected with a hot inert gas on the lateral border of the anode of a coiled generator. In principle, the technique of pulverization with an electric arc of these same metals seems to present the same difficulty because of thermic shock with the other plastic components of the generator.